Investigation of the Iron‐Sulfur Clusters in Some Mitochondria1 Mutants of Saccharomyces cerevisiae

We have undertaken a systematic analysis of the iron‐sulfur clusters present in strains of Saccharomyces cerevisiae carrying mutations in the COB‐BOX region of the mitochondrial DNA coding for cytochrome b , and in OXI loci of the mitochondrial DNA coding for subunits of cytochrome oxidase. The iron‐sulfur clusters and cytochromes in complexes II and III of the yeast mitochondrial respiratory chain have been largely identified. Mutant phenotypes are defined in terms of succinate dehydrogenase and cytochrome c reductase activities, low‐ temperature absorption spectra and electron paramagnetic resonance spectra. From quantitative comparison of iron‐sulfur clusters in mitochondrial preparations of BOX , OXI and rho° mutants, we have made the following two sets of observations. (a) Among the components of complexIII, the appearance of Rieske's iron‐sulfur cluster seems to be correlated only to the presence or absence of the cytochrome b ; moreover Rieske's cluster is absent even when polypeptide fragments of apocytochrome b are known to be synthesized. This observation constitutes a very interesting example illustrating perturbation of the incorporation of a cytoplasmically made protein (Rieske's protein) by structural alterations of a mitochondrially made protein (cytochrome b ), both being assembled within the same multiprotein complex. The situation differs with another cytoplasmically‐made protein (cytochrome c 1 ). (b) Clusters S‐1, S‐2 and S‐3 associated with complex II are present, irrespective of the type of mutation, at about ∼ 10‐20 % of the level of that in the wildtype species; this partial repression could be a result of the respiratory deficiency of these mutants.